1. Field of the Invention
The present invention relates to surface acoustic wave devices used for surface acoustic wave resonators, surface acoustic wave filters, and the like, and particularly to high-frequency, high-voltage surface acoustic wave devices, and to a production method thereof.
2. Description of the Related Art
An interdigital electrode transducer for a surface acoustic wave (SAW) device generally includes an elemental Al or Al-based alloy layer which has low electric resistance and a small mass. However, since such layers are not sufficiently resistant to stress migration, hillocks or voids may be formed therein when this happens. Short circuits may be generated when a high voltage is applied to the device with the result that the device will be destroyed.
In order to solve this problem, the following methods have been proposed:
(1) depositing an electrode layer composed of a metal, such as Ti, which ameliorates the orientation of an elemental Al or Al-based alloy layer on a base of the SAW device and then depositing the elemental Al or Al-based alloy layer on the electrode layer; and
(2) sandwiching an elemental Cu layer between elemental Al or Al-based alloy layers.
Dry etching, such as RIE or ion milling, has been used for patterning the above electrode layer. Ion milling is effective in etching elemental Cu layers and Al-based alloy layers containing Cu but is not effective in etching Ti layers since the rate of etching elemental Ti layers by ion milling is extremely slow. For this reason, it is necessary to remove the elemental Ti by Reactive Ion Etching (RIE) after the elemental Cu layers and Al-based alloy layers containing Cu are removed by ion milling. Because a plurality of etching techniques are required, several drawbacks arise: complexity of the process; the need for a large investment in different etching equipment; and an increase in production cost of the surface acoustic wave device due to the complexity of the process.
Furthermore, when producing an interdigital electrodes transducer having a width of 1 xcexcm or less, the following problems arise: deterioration of device characteristics due to the damage caused in the piezoelectric substrate during processing; variation in etching; and low yield.
In order to solve the above problem, it is an object of the present invention to provide a method for producing a surface acoustic wave device by patterning electrode layers effectively and to provide a surface acoustic wave device, to which high-frequency or high-voltage is applied, according to the above production method.
According to one aspect of the present invention, a method for producing a surface acoustic wave device is provided. The surface acoustic wave device includes a piezoelectric substrate, and an interdigital electrodes transducer arranged on the piezoelectric substrate and a first electrode layer including elemental Ti or a Ti-based alloy and a second electrode layer including elemental Cu or an Al-based metal material containing at least 2% by weight of Cu. The method comprises:
forming a resist pattern corresponding to the interdigital electrodes transducer onto the piezoelectric substrate;
forming the first electrode layer onto both the piezoelectric substrate and the resist pattern by thin-film deposition;
forming the second electrode layer onto the first electrode layer, the second electrode layer including elemental Cu or an Al-based metal material containing at least 2% by weight of Cu (the second electrode layer can have a multilayered structure); and
simultaneously removing the resist pattern and the portions of the first and second electrode layers located on the resist pattern to pattern the first electrode layer and the second electrode layer.
In the production method of the surface acoustic wave device according to the present invention, the first electrode layer and the second electrode layer are preferably deposited on both the resist pattern and the piezoelectric substrate through the openings of the resist pattern, and then the unnecessary portions of the first and second electrode layers located on the resist pattern are simultaneously removed along with the resist pattern. The foregoing lift-off procedure allows the first and second electrode layers to be patterned more effectively than the conventional patterning which uses a plurality of etching techniques.
In the above production method, the first electrode layer includes elemental Ti or a Ti-based alloy and the second electrode layer is arranged on the first electrode layer. As a result, orientation in the second electrode layer is excellent and the formation of hillocks or voids is suppressed. Accordingly, electrode layers having high voltage resistance are produced.
The second electrode layer may be a multilayer structure, e.g., one formed by sandwiching a layer including elemental Cu or an Al-base alloy containing at least 2% by weight of Cu between Al electrode layers. Other multilayer structures can also be used.
The present invention is also directed to a production method an interdigital electrodes transducer whose main part has a width of 1 xcexcm or less.
According to conventional production methods, it is difficult to produce surface acoustic wave devices having interdigital electrodes transducers of whose the main part has a width of 1 xcexcm or less because the piezoelectric substrates are damaged during the production process. In contrast, the production method of the present invention effectively provides a surface acoustic wave device having an interdigital electrodes transducer whose main part has a width of 1 xcexcm or less without causing the following disadvantages: damage to the piezoelectric substrate, deterioration of device characteristics, low yield due to variation in etching preciseness.
The present invention is also directed to a surface acoustic wave device comprising:
a piezoelectric substrate;
an interdigital electrodes transducer formed onto the piezoelectric substrate by simultaneously patterning a first electrode layer and a second electrode layer formed on the first layer using a lift-off procedure, wherein the first electrode layer includes elemental Ti or a Ti-based alloy, and the second electrode layer includes elemental Cu or an Al-based metal material containing at least 2% by weight of Cu or the second electrode layer being arranged on the first electrode layer.
The surface acoustic wave device of the present invention is preferably produced by a lift-off procedure in which the first electrode layer and the second electrode layer are patterned at the same time. As a result, formation of hillocks or voids on the piezoelectric substrate is suppressed and the interdigital electrodes transducer has substantially the same dimensions as the design value and includes a second electrode layer having an excellent orientation. Accordingly, the surface acoustic wave device including the interdigital electrodes transducer has the desired characteristics and high reliability.
The present invention is also directed to a surface acoustic wave device having a main part of a width of 1 xcexcm or less.
The surface acoustic wave device is reliably produced without causing decrease in yield due to variation in etching preciseness because the surface acoustic wave device has the above structure.